The present invention relates to a process and an apparatus for the in-line inspection of the depth of a weld by means of a pulse beam. In line inspection is understood to mean that inspection takes place at the same time as the welding operation. It is more particularly used in inspecting shallow welds.
The Expert is aware of numerous methods for inspecting or controlling the depth of a weld. The most frequently encountered methods are based on ultrasonics, eddy currents, potentiometry and absorption spectrometry.
In the ultrasonic method, it is necessary to use a frequency such that the propagation of the ultrasonics, without exaggerated losses, requires the use of a fluid called a "coupling fluid". Thus, the ultrasonic method cannot be used in the case of an in-line inspection, because the coupling fluid necessary for this method is destroyed by the welding beam.
The eddy current method and the potentiometric method for inspecting the depth of a weld have common disadvantages. These two methods make it necessary to move close to the welding point of the probes, in the case of the eddy current method, or the electrodes in the case of the potentiometric method. The precise positioning of these probes is difficult and can also be prejudicial to the weld.
The method for inspecting the depth of a weld by the absorption of a line operates in the following way. A monochromatic radiation source is placed in a welding enclosure containing the part to be welded and the welding beam and said monochromatic radiation is directed against the welding plasma. This monochromatic source is generally a hollow cathode lamp. A fraction of the emitted radiation is absorbed by the particles present in the welding plasma. These particles are supplied to the part to be welded (tracer method) or are present in the part to be welded in the form of impurities (autotracer method). They are released in the welding plasma by the welding beam. The number of particles released and consequently the fraction of the monochromatic radiation absorbed is proportional to the depth of the weld. The depth of the weld is then evaluated by comparison between the level of the collected signal and the level of a reference signal.
The monochromatic radiation source cannot be used continuously. Thus, in this case, the signal collected in the optical means is constituted by the unabsorbed fraction of the monochromatic radiation, on which is superimposed an emission radiation due to the welding plasma. However, the emission of the welding plasma has a much higher level than the emission of the monochromatic radiation. Consequently, the useful signal disappears in a background noise and cannot be used.
In order to obviate this problem, it is necessary to break down the monochromatic signal into a pulse beam and to carry out a synchronous detection. In the case of welding by a continuous beam, such as an electron beam, the frequency of the monochromatic signal is e.g. 500 Hz. In the case of welding by a pulse beam, it is at the level of each welding pulse that the monochromatic source must emit a large number of pulses. For a welding pulse beam, such as a laser beam supplying e.g. pulses lasting 4 ms every 100 ms, the frequency of the pulses of the monochromatic source must be roughly a few kilohertz so that, during each welding beam pulse, the monochromatic pulse source emits about 100 pulses. The Expert does not have simple means making it possible to easily realise such a monochromatic pulse beam.
No known inspection method consequently makes it possible to carry out an in-line inspection of the depth of a weld by a pulse beam and which is simple, reliable and accurate. The object of the invention is to provide a process and an apparatus enabling such an inspection to be carried out.